J2ObjC
Google (via Michael Jurewitz):
J2ObjC is an open-source command-line tool from Google that translates Java code to Objective-C for the iOS (iPhone/iPad) platform. This tool enables Java code to be part of an iOS application’s build, as no editing of the generated files is necessary. The goal is to write an app’s non-UI code (such as data access, or application logic) in Java, which is then shared by web apps (using GWT), Android apps, and iOS apps.
There’s some documentation about how it works, e.g. they implemented auto boxing and unboxing and translate overloaded Java methods.
